A liquid crystal panel driver LSI and a CCD driver LSI, for example, are operated at a power supply voltage of 10V or higher, and therefore high breakdown voltage transistors having a breakdown voltage of 20V or higher are normally required. On the other hand, low breakdown voltage transistors are used in internal control logic sections that need to be small in size and operated at high speeds. The wells where high breakdown voltage transistors are formed tend to be made deeper, and their surface impurity concentration lowered in order to secure the well breakdown voltage. In contrast, wells where low breakdown voltage transistors are formed are tend to be made shallower, and their surface impurity concentration increased in order to reduce the element size and to achieve higher speeds. For this reason, high breakdown voltage transistors are formed in a chip that is independent from a chip for low breakdown voltage transistors, and known to be formed as an externally mounted circuit.
An object of the present invention is to provide a method for manufacturing a semiconductor device having a high breakdown voltage transistor and a low breakdown voltage transistor with different driving voltages provided in a common substrate.